selftests/bpf: Migrate tcpbpf_user.c to use BPF skeleton

Update tcpbpf_user.c to make use of the BPF skeleton. Doing this we can
simplify test_tcpbpf_user and reduce the overhead involved in setting up
the test.

In addition we can clean up the remaining bits such as the one remaining
CHECK_FAIL at the end of test_tcpbpf_user so that the function only makes
use of CHECK as needed.

Signed-off-by: Alexander Duyck <alexanderduyck@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/160443931155.1086697.17869006617113525162.stgit@localhost.localdomain